As an example of a discharge lamp lighting device, there is one with a configuration shown in FIG. 11. This discharge lamp lighting device 100 is composed of a power supply circuit unit 101 and a lighting control circuit unit 102. The power supply circuit unit 101 is, for example, a chopper circuit. The power supply circuit unit 101 is composed of a direct current power supply E, a load 109, a switching element Q, a resistor R for detecting a current flowing through the switching element Q, a power transformer T, a diode D, a capacitor C, and a load voltage detector 103 that detects a voltage of the load 109. The switching element Q of the power supply circuit unit 101 performs a switching operation by a drive signal outputted from a drive controller 108 of the lighting control circuit unit 102 in response to a state (power, voltage and the like) of the load 109.
The lighting control circuit unit 102 is composed of a zero cross detector 104 that is connected to a secondary side of the power transformer T and detects that a secondary current has turned to zero, a current detector 105 that compares, in terms of magnitude, the current flowing through the switching element Q and a reference value Is corresponding to power of the load 109 with each other, a reference arithmetic unit 106 that outputs the reference value Is in response to the power of the load 109, an A/D converter 107 that performs A/D conversion for the load voltage detected by the load voltage detector 103, and the drive controller 108 that controls on/off of the switching element Q. The drive controller 108 is composed, for example, by having an RS flip-flop and the like.
With regard to a current of the power transformer T, during an on-time Ton while the switching element Q is being turned on, as shown in FIG. 12A, energy is charged to the power transformer T, and the current of the power transformer T is increased. During an off-time Toff while the switching element Q is being turned off, as shown in FIG. 12B, a regenerative current that releases the energy stored in the power transformer T flows through the capacitor C and the diode D, and is gradually decreased. When the energy stored in the power transformer T is entirely released, the current of the power transformer T turns to zero.
When it is detected that the energy stored in the power transformer T has turned to zero by the zero cross detector 104 of the lighting control circuit unit 102, as shown in FIG. 12C, the zero cross detector 104 inputs a turn-on signal of the switching element Q to a set input terminal SET of the drive controller 108. The drive controller 108 turns on the switching element Q.
The load voltage Vla is detected by the load voltage detector 103, and the load voltage Vla concerned is monitored by the reference arithmetic unit 106 through the A/D converter 107, whereby the reference voltage arithmetic unit 106 outputs the reference value Is corresponding to the power of the load 109. The current detector 105 compares a value detected by the resistor R from the current flowing through the switching element Q and a value of the reference value Is with each other. When the detected value of the current flowing through the switching element Q becomes more than the reference value Is, as shown in FIG. 12D, the current detector 105 outputs a detection signal to a reset input terminal RESET of the drive controller 108, and the drive controller 108 turns off the switching element Q. FIG. 12E is a voltage output waveform of an output terminal Dout of the drive controller 108. In such a way, the discharge lamp lighting device can supply the appropriate power corresponding to the load voltage Vla, and moreover, can control the switching element Q to turn on at timing when the current flowing through the transistor T makes a zero cross.
In Patent Document 1 (Japanese Patent Laid-Open Publication No. 2004-178925), there is described a discharge lamp lighting device that turns off a chopper when the current flowing through the chopper becomes a predetermined reference value or more and turns on the chopper when energy release of a chopper coil is detected. This discharge lamp lighting device includes an off-time timer circuit that counts an off-time of the chopper, and includes a zero cross detector that, when a predetermined time is counted by the off-time timer circuit before detecting the energy release of the chopper coil, detects the energy release of the chopper coil while turns on the chopper at timing when the predetermined time is counted.
In the case of controlling the switching element Q to turn on at the timing of the zero cross of the current by the power supply circuit unit using the chopper circuit, which is shown in FIG. 11, the discharge lamp lighting device requires an operation to detect that the energy stored in the power transformer T has turned to zero. Accordingly, this discharge lamp lighting device requires zero cross detection using a secondary winding of the power transformer T, and has a problem that the number of parts is increased, bringing up a cost increase.
Moreover, an operation frequency in the case of controlling the switching element Q to turn on by the zero cross detection is decided by self-excitation by an input voltage Vin of the direct current power supply E, the load voltage Vla, load power, and an inductance value of the power transformer T. Accordingly, when the load voltage Vla of a discharge lamp is changed with time, a switching frequency is changed as shown by a broken line of FIG. 5. When noise of the switching frequency is superimposed on the load current, and a ripple component by the noise is superimposed on a frequency-prohibited range intrinsic to the load, there is also a problem that such a ripple cannot be avoided.
In particular, when the load 109 is a high-voltage discharge lamp, as the frequency-prohibited range intrinsic to the load, there is a range where an acoustic resonance phenomenon occurs, and there is an apprehension to bring up instability, fading, flickering and the like of arcs.
The present invention has been made in consideration for the points as described above. It is an object of the present invention to control the operation frequency of the switching element in response to a state of the discharge lamp without bringing up the cost increase of the discharge lamp lighting device, thus making it possible to control such on-timing of the switching element to be timing when the regenerative current turns to zero.